Nucleosomes are histone multimers around which approximately 146 base pairs of DNA are wrapped. They serve at least two functions in eukaryotic cells: compacting the DNA into chromatin and exerting a general repressive effect on gene expression. Considerable attention has been focused recently on the mechanisms by which cells alter chromatin structure to allow the cellular transcriptional machinery to gain access to genes and their promoters. Nucleosomes have been reported to form on the genomes of mammalian viruses, e.g., SV40. In the case of Herpes Simplex Virus Type 1 (HSV-1), published reports indicate that latent virus DNA is nucleosome associated. On the other hand, it has been reported that nucleosomes do not form on HSV-1 during the lytic replication cycle. However, our data strongly suggest that, in fact, nucleosomes do form on HSV-1 DNA during lytic replication. This has important implications for understanding the mechanisms of HSV-1 gene regulation. The goal of this proposal is to characterize the interactions of nucleosomes with the viral genome. Three specific aims are proposed. In the first aim, quantitative assays will be used to determine the fraction of HSV-1 DNA that is nucleosome associated. In the second aim, the organization of nucleosomes in the vicinity of the viral promoters will be determined. In the final aim, the effect of viral regulatory proteins on promoter chromatin structure will be determined. It is anticipated that this work will substantially enhance our knowledge of HSV-1 gene regulation, with important implications for our understanding of HSV-1 latency and pathogenesis, development of novel antiviral drugs, and possible use of HSV-1 as a vector for gene therapy.